Map from lockfile dep_path to the absolute on-disk directories
where that package ended up. Most entries have exactly one path;
packages whose name conflicts with a shallower version end up
duplicated across multiple parent node_modules/ directories so
each gets its own on-disk copy.
Build a dep_path → absolute on-disk target map for every
LocalSource::Link in the graph. Returned None when the graph
has no link entries (vast majority of installs), so the materialize
hot path can short-circuit without a per-dep lookup.
xx::file::mkdirp wrapped with the linker’s Error::Xx conversion.
Every materialize pass calls this before creating a symlink /
junction, so the lossy .to_string() wrap lives in exactly one
place.